Yup, as a silver until fairly recently I have far too much experience of getting up just before T-72 for very early flights in 3 days to do exactly this. Probably worked about 30-40% of the time.

And I think the answer to your other question is no, based on one/two experiences where I knew that passenger was a gold, and I guess a general assumption that blocks still hold (providing there's space) up until check-in closes. Which I also base on using it to block a middle seat of 3 on a return trip to Vegas when I became gold, and the only free seat on both flights was the sneaky middle one between us. But again I'm only guessing.